a) Field of the Invention
The present invention relates to an optical low-pass filter used to eliminate moire in television cameras, electronic cameras and fiberscopes provided with a solid state image pickup device.
b) Description of the Prior Art
In television cameras using a solid state image pickup device, a difficulty is encountered in which components of frequency higher than that (Nyquist rate) of half of the sampling frequency depending on the pitch of an array of pixel elements are included in a picture image, a false signal is generated with a resultant deterioration of the picture image. Such a false signal is generally termed "aliasing", "moire", etc. and in particular, if the high frequency components, such as stripe patterns with a regular array, are included in the picture image, moire will cause the picture image to be very unsightly.
The high frequency components included in an object image must be previously eliminated in an objective optical system, namely, before reaching the solid state image pickup device. In the past, an optical low-pass filter employing birefringent elements has been used. This optical filter is usually composed of one or more artificial crystal. Characteristics of the filter using one crystal plate, that is, an absolute value MTF (modulation transfer function) of optical response function, describe a curve of cos .vertline.cos .theta..vertline. as drawn with a solid line in FIG. 1. Then, this curve causes the frequency of a first trap point (a point where the response is zero) to coincide with the Nyquist rate. The curve, however, rises up rapidly on the high frequency side from the Nyquist rate side, so that an effect of moire elimination is weakened. For the moire elimination effect to be lessened in the vicinity of the position of the Nyquist rate, a crystal plate is overlapped with another one having the same filter characteristics in such a manner that a 1/4-wave plate is sandwiched between them. MTF represents a curve of .vertline.cos.sup.2 .theta..vertline. as indicated with dotted line in FIG. 1 and its rising becomes relatively smooth in the vicinity of the trap point, with the result being that filter characteristics having an excellent moire elimination effect are available.
If, however, the filter consisting of such crystal plates is used in an electronic endoscope whose objective optical system and solid state image pickup device are incorporated in its tip portion, the objective optical system is liable to be lengthened due to the thickness of the crystal plate filter and, as a result, compaction of the tip portion of the endoscope has been considerably prevented. Specifically, the optical low-pass filter using one crystal plate as well as plural ones has a high rate of thickness bearing in the objective optical system of the endoscope and in particular, an objective optical system inclucing a prism in an optical path as shown in FIGS. 2A and 2B makes further compaction difficult. Also, in FIGS. 2A and 2B, reference numeral 1 designates an objective lens, 2 a crystal plate filter, 3 a right-angled triangle prism, and 4 a solid state image pickup device.
Here, as a countermeasure for solving the above difficulty, an optical filter described in Japanese Patent Preliminary Publication No. Sho 61-223802, for example, has been proposed. This optical low-pass filter is an optical element having a half mirror surface and a mirror surface provided parallel to each other in slightly spaced relation and arranged in an image forming optical path from an object to a solid state image pickup device so that the half mirror surface is provided on the outside of the element. Light coming from the object is obliquely incident on the half mirror surface and the mirror surface. In this configuration, light is reflected from the half mirror surface and light traversing the half mirror surface, reflected from the mirror surface slightly spaced, and traversing again the half mirror surface form a twin image of the object slightly spaced on a light receiving surface of the solid state image pickup device. The optical filter of the type, therefore, has the same function as in the case where a crystal plate forms the twin image of the object slightly spaced due to the ordinary ray and extraordinary ray and bears spatial frequency characteristics similar to the crystal plate.
However, in this optical filter, many ghost images are formed by multiple reflections occurring between the half mirror surface and the mirror surface in such a manner that a part of light reflected from the mirror surface is reflected from the half mirror surface, reflected light is reflected again from the mirror surface, a part of the light traverses the half mirror surface, the remainder is reflected from the half mirror surface and then from the mirror surface again, . . . Such properties have a great effect on the response of the optical element and in this conventional example, the above respect has not been discussed completely, with resultant lack of practical use.
Also, with coloring of a picture image, there has been a tendency for television cameras to use the system that a color coded mosaic filter is arranged on the image pickup device to form a color picture image from a signal read out. In order to form a picture image with a high degree of quality through an image pickup device with a preset number of pixel elements in particular, a system has been recently developed such that, instead of the color mosaic filter comprising conventional primaries uniformly distributed, a color mosaic filter composed of Cy (Cyan=Blue+Green), Green, and Ye (Yellow=Red+Green) as shown in FIG. 3 is employed to derive a G signal also used as a luminance signal from all pixel elements. That is, in this color mosaic filter, the luminance signal (G signal) is read out from all pixel elements, while B and R signals are individually read out from alternate pixel element. An optical low-pass filter most suitable for such a color mosaic filter has been proposed by U.S. Pat. No. 4,575,193. The feature of this optical low-pass filter is that its response has different wavelength dependency in accordance with wavelengths of light incident on the filter. Specifically, the frequency of the first trap point relating to G light is set to be twice that of B and R light and the filter has a wide band with respect to the G light and a narrow band to the B and R light. However, since this optical low-pass filter utilizes wavelength dependency of phase retardation of a wave plate, it has the property that the response relating to each color of R, G and B gradually changes in accordance with wavelengths. As a result, even in the wavelength band specified as a primary color as in each of R, G and B, the response will change unlimitedly in accordance with wavelengths. Accordingly, the optical low-pass filter of the type is not constructed so as to be optimized to various properties of a color mosaic filter using a complementary color system, that is this type of low-pass filter brings about color moire and thus has performance insufficient to secure good picture quality in practical use.